Polyoxometallates and heteropolyacids, both in general and those which can be used to prepare some of the catalysts used in our invention, and their preparation are described in Pope et al., Heteropoly and Isopoly Oxometalates, Springer-Verlag, New York (1983).
Polyoxometallates and heteropolyacids consist of a polyhedral cage structure or framework bearing a negative charge (e.g., [PMo.sub.12 O.sub.40 ].sup.-3 ; [P.sub.2 Mo.sub.18 O.sub.62 ].sup.-6) which is balanced by cations that are external to the cage. If the cations are protons, then the compound is a heteropolyacid (HPA) (e.g., H.sub.6 [P.sub.2 Mo.sub.18 O.sub.62 ]). If the cations were not all hydrogen, but either metals such as an alkali metal, potassium, sodium, or lithium, as in K.sub.6 P.sub.2 W.sub.18 O.sub.62, or ammonium, as in (NH.sub.4).sub.6 P.sub.2 Mo.sub.18 O.sub.62, then it is referred to as a polyoxometallate (POM). In earlier patents, we have used the term "polyoxoanion" to describe compounds in which some or all of the cations are not hydrogen (e.g., K.sub.3 PMo.sub.12 O.sub.40); in the present case, however, these compounds are referred to as polyoxometallates and the term polyoxoanion is reserved for describing the anionic cage-like portion of the compound (e.g., [P.sub.2 Mo.sub.18 O.sub.62 ].sup.-6).
As described in Pope et al., supra, heteropolyacids and polyoxometallates are cage-like structures with a primary, generally centrally located atom(s) surrounded by a cage framework, which framework contains a plurality of metal atoms, the same or different, bonded to oxygen atoms. The central element of heteropolyacids and polyoxometallates is different from metal atoms of the framework and is sometimes referred to as the "hetero" element or atom; the condensed coordination elements are referred to as the "framework" elements or metals. The framework metal atoms are ordinarily transition metals. As described by Pope et al., supra, the majority of heteropolyacids and polyoxometallates have a centrally located heteroatom ("X") usually bonded in a tetrahedral fashion through four oxygen atoms to the "framework" metals ("M"). The framework metals, in turn (i) are usually bonded to the central atom in an octahedral fashion through oxygens ("O"), and (ii) are bonded to four other framework metals through oxygen atoms and (iii) have a sixth non-bridging oxygen atom known as the "terminal oxygen" atom. This can be illustrated as shown below: ##STR1##
The principal framework metal, M, is effectively limited to only a handful of metals including molybdenum, tungsten, vanadium, niobium and tantalum. According to Pope et al., supra, this is due to the necessary condition that suitable metals have appropriate cation radius and be good oxygen p.pi.-electron acceptors. Among the successful candidates, molybdenum and tungsten share a common feature; namely, the expansion of valences of their metal cations from four to six. The coincidence of these characteristics allow these metals to form stable heteropolyacids and polyoxometallates.
Conventional heteropolyacids (and polyoxoanions thereof) can be described by the general formula H.sub.e (X.sub.k M.sub.n O.sub.y).sup.-e. In this formula, X, the central atom, is frequently phosphorus. However, other suitable central atoms include Group IIB-VIB elements, such as antimony, silicon and boron. Further, the subscript k is preferably 2, but can be from 1 to 5. M is molybdenum, tungsten, or vanadium and n will vary from 5-20. The subscript y may be as low as 18 or as high as 62. The notation e is the negative charge on the (X.sub.k M.sub.n O.sub.y) polyoxoanion and will vary from case to case, but e is always the number of protons needed to balance the formula. In a typical such heteropolyacid, k=2, n=18 and y=62, as in H.sub.6 P.sub.2 Mo.sub.18 O.sub.62 and the polyoxometallate H.sub.2 (VO).sub.2 [P.sub.2 Mo.sub.18 O.sub.62 ].
As described in Pope et al., supra, heteropolyacids are known to exist in a variety of structures including the Keggin, Wells-Dawson and Anderson structures. The different structures correspond to the specific geometry of particular heteropolyacid compositions and vary according to the coordination chemistry and atomic radii of the metals present. These compounds may be substituted at various framework sites as disclosed, inter alia, in our prior patents. The present invention focuses on compounds of the Wells-Dawson type structure.
In our U.S. Pat. No. 4,803,187, issued Feb. 7, 1989, we taught how to prepare heteropolyacids and polyoxometallates with random substitution of framework metals, such as H.sub.7 (PMo.sub.8 V.sub.4 O.sub.40); K.sub.6 (SiMo.sub.11 MnO.sub.39) and K.sub.5 (PW.sub.11 VO.sub.40). The preparation of framework-substituted heteropolyacids or polyoxometallates as described in our U.S. Pat. No. 4,803,187, supra, is adequate for random substitution, but will not provide the regiospecific, trilacunary substitution as described in our U.S. Pat. No. 4,898,989, supra; i.e., replacement of three M in a single, triangular face with three M'. The teaching of U.S. Pat. No. 4,803,187 and U.S. Pat. No. 4,898,989 is incorporated for all purposes by reference herein.
As described in Pope et al., supra, heteropolyacids and polyoxometallates have found a variety of applications. In the area of catalysis, Keggin ion catalysts have been used in connection with the oxidation of propylene and isobutylene to acrylic and methacrylic acids, oxidation of aromatic hydrocarbons; olefin polymerization; olefin epoxidation; and hydrodesulfurization processes. See, for example, M. Ai, "Partial Oxidation of n-Butane with Heteropoly Compound-based Catalysts", Proceedings of the 18th International Congress on Catalysis, Berlin, 1984, Verlag Chemie, Vol. 5, page 475; Lyons et al., U.S. Pat. No. 4,803,187, issued Feb. 7, 1989; Lyons et al., U.S. Pat. No. 4,859,798, issued Aug. 22, 1989; Ellis et al., U.S. Pat. No. 4,898,989, issued Feb. 6, 1990; Lyons et al., U.S. Pat. No. 4,916,101, issued Apr. 10, 1990; Ellis et al., U.S. Pat. No. 5,091,354, issued Feb. 25, 1992; and Shaikh et al., U.S. Pat. No. 5,334,780, issued Aug. 2, 1994; each of which is incorporated herein by reference.
Framework-substituted Keggin heteropolyacids have been disclosed as catalysts for oxidation of aldehydes, cyclohexene and cyclohexane, and for hydrogen peroxide decomposition. N. Mizuno et al., "Synthesis of [PW.sub.9 O.sub.37 {Fe.sub.3-x Ni.sub.x (OAc.sub.3 }].sup.(9+x)- (x=predominantly 1) and Oxidation Catalysis by the Catalyst Precursors", J. Mol. Cat., 88, L125-31 (1994); and Wu et al., "Catalytic Behavior of Metal Ions Located at Different Sites of Heteropoly Compounds", Catalysis Letters, 23, 195-205 (1994).
Non-framework substituted Keggin polyoxometallates and heteropolyacids are known in the art as catalysts for oxidation of isobutane to methacrylic acid and methacrolein. W. Ueda et al., "Catalytic Oxidation of Isobutane to Methacrylic Acid with Molecular Oxygen over Activated Pyridinium 12-Molybdophosphate", Cat. Lett., 261-265 (1997); N. Mzuno et al., "Catalytic Performance of Cs.sub.2.5 Fe.sub.0.08 H.sub.1.26 PVMo.sub.12 O.sub.40 for Direct Oxidation of Lower Alkanes", J. Mol. Catal., A, 114, 309-317 (1996); F. Trifiro, "Reactivity of Keggin-type Heteropolycompounds in the Oxidation of Isobutane to Methacrolein and Methacrylic Acid: Reaction Mechanism", J. Mol. Catal., A, 114, 343-359 (1996); N. Mizuno et al., "Direct Oxidation of Isobutane into Methacrylic Acid and Methacrolein over Cs.sub.2.5 Ni.sub.0.08 -substituted H.sub.3 PMo.sub.12 O.sub.40 ", J. Chem. Soc., Chem. Commun., 1411-1412 (1994); S. Yamamatsu et al., "Process for Producing Methacrylic Acid and Methacrolein", European Patent Specification Publication No. 0 425 666 BI, Application No. 89905775.6 filed May 22, 1989, Date of publication of patent specification Apr. 13, 1994; S. Yamamatsu et al., "Method for the Fabrication of Methacrylic Acid and/or Methacrolein", Japanese Patent Application Public Disclosure No. H2-42034, Feb. 13, 1990; S. Yamamatsu et al., U.S. Pat. No. 5,191,116, issued Mar. 2, 1993; K. Nagai et al., Process for producing methacrylic acid and methacrolein by catalytic oxidation of isobutane", European Patent Application Publication No. 0 418 657 A2, Application No. 90117103.3, filed Sep. 5, 1990 by Sumitomo Chem.Ind.KK (published Mar. 27, 1991); T. Jinbo et al., "Method for the Manufacture of Acroleic Acid or Acrylic Acid, and Catalysts Used Therein", Japanese Patent Application Public Disclosure No. H6-218286, Aug. 9, 1994; M. Ai, "Partial Oxidation of n-Butane with Heteropoly Compound-based Catalysts", Labo. Resources Utiliz., Tokyo Inst. Tech., Yokohama, Japan, 8th International Congress on Catalysis Volume V: Cluster-derived catalysts Active phase support interactions Catalysts for synthesis, of Chemicals, Verlag Chemie, Berlin, pages V475-V486 (1984); G. Centi et al., "Selective Oxidation of Light Alkanes: Comparison between Vanadyl Pyrophosphate and V-Molybdophosphoric Acid", Catal.Sci.Technol., Proc. Tokyo Conf., 1st Meeting, 1990, 225-30; N. Mizuno et al., "Catalytic Performance of Cs.sub.2.5 Fe.sub.0.08 H.sub.1.26 PVMo.sub.11 O.sub.40 for Direct Oxidation of Lower Alkanes", J. Mol. Catal., A, 114, 309-317 (1996); M. Ai, "Oxidation of Propane to Acrylic Acid", Catalysis Today, 13 (4), 679-684 (Eng.) (1992); N. Mizuno et al., Applied Catalysis A: General, 128, L165-L170 (1995); Ueda et al., Chemistry Letters, 541, 2 (1995); Cavani et al., Catalysis Letters, 32 215-226 (1995).
The references cited above primarily employed non-framework substituted Keggin-type heteropolyacids as catalysts in manufacture of unsaturated carboxylic acids, for example acrylic acid and methacrylic acid, from alkanes, for example propane and isobutane. There is no known use of Wells-Dawson-type heteropolyacids for catalysis of these reactions.
Wells-Dawson-type heteropolyacids are more difficult to prepare than the Keggin compounds. This may explain the paucity of published works regarding their activity. In fact, work relating to Wells-Dawson structures is primarily limited to their use for certain homogeneous liquid-phase reactions (Hill, et al., Coord.Chem.Rev., 143, 407 (1995)) and in the decomposition of hydrogen peroxide (Wu, et al., Cat.Lett., 23, 195 (1994)). Comuzzi et al., Cat.Lett., 36, 75 (1996), investigated the gas-phase oxidative dehydrogenation of isobutane to isobutene catalyzed by K.sub.6 P.sub.2 W.sub.18 O.sub.62, a Wells-Dawson-type phosphotungstate. However, despite the literature on Keggin-type compounds, there has been no disclosure of the use of the acid form of the Wells-Dawson-type compounds (i.e., H.sub.e (P.sub.2 M.sub.18 O.sub.62).sup.-e) or use of Wells-Dawson-type phosphomolybdates (e.g., K.sub.6 P.sub.2 Mo.sub.18 O.sub.62), for example, for the heterogeneous gas-phase oxidation of alkanes to unsaturated carboxylic acids.
Given the value and industrial importance of acrylic acid and methacrylic acid, it has been recognized that the one-step conversion of alkanes to unsaturated carboxylic acids would be a useful process with important commercial applications, provided that sufficient yield can be obtained. To date, no efficient catalysts have been developed for the commercial production of acrylic acid from propane or methacrylic acid from isobutane. As a result, acrylic acid is manufactured from propylene, a raw material which is over three times more expensive than propane.
The process of the present invention provides such a one-step process for the conversion of alkane to carboxylic acid catalyzed by Wells-Dawson type HPAs. These catalysts have been found to yield superior results to Keggin-type HPAs having similar metals. Through electrochemical experiments, we have demonstrated that Wells-Dawson HPAs have superior redox properties to Keggin HPAs. At the same time, we have found that Wells-Dawson HPAs are more efficient catalysts for the oxidation of alkanes to .alpha.-.beta.-unsaturated oxidation products than Keggin HPAs in comparable experiments. These advantages make the process more attractive than the prior art processes for practical use and potential commercial interest.